The present invention relates to supports for gas turbine nozzle stages having closed circuit cooling, for example, steam cooling, and particularly relates to a hook for supporting a closed circuit, steam cooled nozzle stage segment from a fixed portion of the turbine casing.
Closed circuit steam cooled nozzle stages for a gas turbine typically have an annular array of nozzle vane segments each having inner and outer bands with one or more nozzle vanes extending generally radially between the bands. To provide a closed circuit cooling system, each of the bands has a chamber for containing the cooling medium, e.g., steam for cooling the walls of the nozzle stage. The vane between the chambers is divided into cavities and the cooling steam flows from the outer chamber through the cavities for cooling the vane and into the chamber of the inner band for cooling the inner wall. The spent cooling steam then flows through the inner band chamber generally radially outwardly through one or more cavities of the vane to a cooling steam exhaust.
More particularly, as illustrated in U.S. Pat. No. 5,634,766, of common assignee herewith and for each nozzle segment, the outer band comprises an outer wall and a radially outward cover defining the outer chamber between the wall and cover. The cooling steam is supplied through an inlet in the cover and through an impingement plate in the chamber for impingement cooling of the outer wall. The cooling steam then flows through apertures in a cast extension of the vane extending through the outer chamber. From the apertures, steam is directed into inserts in one or more flow cavities in the vane for transmitting the steam through apertures in the inserts for impingement cooling the vane walls, particularly the leading edge. The inner band comprises the inner wall and a radially inner cover and receives the spent cooling steam from the vane. The spent cooling steam reverses direction and flows through apertures in an impingement plate in the inner chamber for impingement cooling of the inner wall. The spent cooling steam flows radially outwardly through an insert in another cavity in the vane for impingement cooling and then through the vane extension of the outer band to a steam exhaust outlet.
From the foregoing, it will be appreciated that the closed loop cooling circuit requires a cover and a wall for each of the outer and inner bands to contain the cooling steam. The nozzle stage segments are also hung from the outer fixed casing of the turbine by forward and aft hooks typically formed integrally with the outer wall of the nozzle stage segment. Particularly, the forward hook is cast as an integral extension of the vane extension. However, difficulties in cooling, manufacturing and attaching the nozzle stage segments to the turbine casings occur with that configuration. For example, the vane extension in the outer band has apertures for flowing the cooling medium into the leading edge cavity of the vane. These cooling apertures cause stress because the load support path for the vane and inner band portions of the nozzle stage segment pass through the hot leading edge and fillet. The cooling flow apertures through each vane extension also afford an undesirable pressure loss as the cooling steam flows from the outer band into the vane. Moreover, from a review of U.S. Pat. No. 5,634,766, it will be appreciated that the location of the forward support hook renders insertion of the impingement cooling insert into the leading edge cavity difficult. Further, the integral mounting of the forward hook on the vane complicates the manufacture and assembly of the nozzle stage segment, affording unnecessary complexity and a substantial number of parts necessary to work around the hook that is cast integrally on the nozzle vane extension.
In accordance with a preferred embodiment of the present invention, the mechanical attachment of the nozzle stage segment to the outer fixed casing of the turbine is accomplished by forward and aft hooks on the outer band, with the forward hook being formed integrally with the cover and the aft hook formed integrally with the outer wall. The vane also includes a vane extension between the wall and cover of the outer band to which the cover with the integral hook is secured, e.g., by welding. The vane extension, however, is spaced back from the leading edge of the vane and the leading edge cavity through the vane. In this manner, the load path extends from the hook through the cover to the vane extension whereby stresses on the hot leading edge and fillet are avoided. That is, the load path includes the first rib between opposite side walls and the first and second cavities of the vane for carrying the load of the cantilevered nozzle. The cover and outer wall are secured, preferably by welding, to one another to define the outer chamber forming part of the closed loop cooling circuit. By locating the forward hook on the outer cover, the impingement insert in the first cavity of the vane can be applied directly. Also, the vane extension does not require apertures for flowing cooling steam into the vane cavities which otherwise would stress the load bearing leading edge of the vane. Also, the number and complexity of the parts is significantly reduced. For example, a single impingement plate can be formed and provided in the outer band chamber about the vane extension. Further, the segment casting is greatly simplified.
In a preferred embodiment according to the present invention, there is provided a nozzle stage segment for a gas turbine, comprising inner and outer bands spaced generally radially from one another and a nozzle vane extending between the bands, the nozzle vane having leading and trailing edges, the outer band including a wall for defining a portion of a hot gas flow path through the turbine and an outer cover radially outwardly of the wall defining a chamber with the wall for forming part of a closed loop cooling circuit through the nozzle stage segment, the outer cover having a generally axially forwardly directed hook for structurally attaching the nozzle stage segment to a support on the turbine.
In a further preferred embodiment according to the present invention, there is provided a nozzle stage segment for a gas turbine, comprising inner and outer bands spaced generally radially from one another and a nozzle vane extending between the bands, the nozzle vane having leading and trailing edges, the outer band including a wall, a vane extension extending generally radially outwardly of the wall, and an outer cover radially outwardly of the wall, the outer cover having a generally axially forwardly directed hook for attaching the nozzle stage segment to a support on the turbine, the vane extension and the outer cover being secured to one another to define a structural load bearing path through the outer cover between the hook and the vane.